With the explosion in communication via the Internet in recent years, there has been a corresponding increase in demand for high-speed bandwidth, such as that provided by optical fibers. In fiber optic communication systems, a fiber that carries optical signals contains asymmetries. These asymmetries result in the optical properties of the fiber not being the same in all directions. Thus, the fiber is birefringent, where the material displays two different indices of refraction. This fiber birefringence causes polarization mode dispersion (PMD).
PMD is measured like a vector quantity, where a differential group delay is the magnitude of the vector and the principal state of polarization (PSP) is the direction. There are two PSPs associated with PMD. The two PSPs propagate at slightly different velocities with the distribution of signal power varying with time.
PMD is a time varying stochastic effect. PMD varies in time with ambient temperature, fiber movement, and mechanical stress on the fibers. Compensating for PMD can be difficult because of the time varying nature and randomness of PMD.
PMD has been shown to be an impairment to the transmission of signals over telecommunication optical fiber at line rates of 10 Gbits/s or above over long distances. Though the problem originates in both optical components and the transmission fiber, the ongoing focus has been PMD reduction in the fiber.
Current processes enable designing and manufacturing optical transmission fiber with very low values of PMD. However, not all manufacturers have access to the intellectual property which is critical to successfully and consistently produce fiber with good PMD performance. Hence, the optical fiber market offers a wide spectrum of quality with little differentiation in specifications. A current challenge for optical cable manufacturers and installers is assessing the true PMD quality of the fiber based on information provided by the fiber manufacturer. In particular, a common question is what fiber/cable qualification procedure should be followed to assure good PMD performance in the installed system.
The traditional specifications on PMD have focused on the link design value (LDV) or maximum differential group delay (DGD-max). These metrics have an inherent weakness, however, of being virtually impossible for a customer to verify. It has become apparent that more information on PMD is required, specific to a customers' fiber order and cable type. Acquiring this information involves careful attention to measurement techniques and correlation of fibers as they move from draw towers through the final installed cable product.
Accordingly, there is a need in the art of optical fiber manufacturing and installation for a system which provides predictive PMD throughout the manufacturing and installation process.